If a train is closely approaching a signal , and the signaller decides to put the signal back to danger, the interlocking will start a timer which gives the train time to stop. While that timer is running, the Route will remain set to avoid any potential conflicts. The signal on the signaller's screen will flash red to indicate that approach locking is active, but the physical signal outside will be showing a steady red aspect. Only after the timer has run will the Route release.
Approach locking timers are typically as follows:
Approach Locking can be "comprehensive" or "non-comprehensive". Comprehensive approach locking will only require a signal to time out if there is a train approaching it that would receive an adverse change of aspect when the signal is replaced. Non-comprehensive approach locking will always time out irrespective of whether there are trains approaching or not.
Last edited by GeoffM on 15/09/2016 at 03:01